Laser crystallization of an amorphous silicon film that has been deposited on a substrate, e.g., glass, represents a promising technology for the production of material films having relatively high electron mobilities. Once crystallized, this material can then be used to manufacture thin film transistors (TFT's) and in one particular application, TFT's suitable for use in relatively large liquid crystal displays (LCD's). Other applications for crystallized silicon films may include Organic LED (OLED), System on a Panel (SOP), flexible electronics and photovoltaics. In more quantitative terms, high volume production systems may be commercially available in the near future capable of quickly crystallizing a film having a thickness of about 90 nm and a width of about 700 mm or longer.
Laser crystallization may be performed using pulsed laser light that is optically shaped to a line beam, e.g., laser light that is focused in a first axis, e.g., the short-axis, and expanded in a second axis, e.g., the long-axis. Typically, the first and second axes are mutually orthogonal and both axes are approximately orthogonal to a central ray traveling toward the film. An exemplary line beam for laser crystallization may have a beam width at the film of less than about 20 microns, e.g. 3-4 microns, and a beam length of about 700 mm. With this arrangement, the film can be scanned or stepped in a direction parallel to the beam width to sequentially melt and subsequently crystallize a film having a substantial length, e.g., 900 mm or more.
In some cases, e.g. sequential lateral solidification processes, it may be desirable to ensure that the silicon film is exposed using a beam having an intensity that is relatively uniform across the short-axis and that drops off sharply at the short-axis edges (i.e. a beam having relatively steep, short-axis sidewalls). More specifically, failure to obtain a steep sidewall on the trailing short-axis edge may result in the undesirable crystal quality of new grains near the short-axis edge due to insufficient overlap between adjacent pulses. Also, in some implementations, it may be desirable to have a steep sidewall on the leading short-axis edge to reduce surface variations and provide more consistent lateral growth.
One way to achieve this shape is to focus a laser at a short-axis element, e.g. field stop, which is shaped as an elongated slit that is aligned in the direction of the long-axis. An optic may then be used to produce an image of the short-axis element at the film. With this arrangement, a beam having relatively steep, short-axis sidewalls may be obtained. For the dimensions contemplated above, e.g. a beam width at the film of less than 20 microns, it may be important to control the dimensions of the short-axis element to relatively close tolerances. Establishing the slit shaped element may involve the positioning of two relatively large mass blocks to within approximately 20-100 microns of each other. Thus, this slightest error in positioning can cause the blocks to collide and damage the critical block edges which define the slit shaped element.
Another factor that must be considered when contemplating the crystallization of large films with the concomitant high power laser beams, is the heat generated at the slit shaped short-axis element. Large amounts of heat can distort the critical block edges which define the slit shaped element, and if severe, cause undesirable aberrations in the laser beam.
With the above in mind, Applicants disclose systems and methods for implementing an interaction between a shaped line beam and a film deposited on a substrate.